The present invention relates to a multi-cylinder internal combustion engine, and more particularly to an internal combustion engine fitted with a thermal reactor in its exhaust system.
In order to reduce pollutants, such as nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC), from motor vehicle propelled by an internal combustion engine, a conventional internal combustion engine for motor vehicles has a main combustion chamber to which a lean air-fuel mixture is supplied, an auxiliary combustion chamber to which a rich air-fuel mixture is supplied and in which ignition of the rich air-fuel mixture is effected by means of a spark plug and a torch nozzle connecting the auxiliary combustion chamber to the main combustion chamber to allow a torch flame into the main combustion chamber to initiate combustion of the lean air-fuel mixture therein. With this combustion arrangement lean combustion with relatively low peak combustion temperatures is possible and formation of nitrogen oxides is suppressed and besides exhaust gas resulted from the combustion is sufficiently oxygen rich for self-oxidation of carbon monoxide and hydrocarbons in the exhaust gas, so that if temperature of the exhaust gas is kept sufficiently high enough, oxidation will occur. For enabling the self-oxidation of carbon monoxide and hydrocarbons of the exhaust gas, an exhaust manifold of the internal combustion engine includes a thermal reaction chamber in a central portion thereof and thermally insulated manifold passages leading from an engine head to the thermal reaction chamber. However, the conventional internal combustion engine has a disadvantage that thermal gradient along an exhaust port passage formed in the engine head is great and this may minimize or eliminate reaction within the thermal reaction chamber.